»What should I know about
disease transmission?

Knowing the methods in which a disease is transmitted
is important for implementing proper infection control measures
and large scale prevention campaigns. Each disease has transmission
characteristics based on the nature of the microorganism that
causes it.

The types of transmission described below are not mutually
exclusive. Some diseases, such as anthrax, can be transmitted
in more than one way. Anthrax
can be spread through direct contact to a cut on the skin, producing
cutaneous anthrax. It can also be spread through airborne spores
which are inhaled, producing a more serious type of infection.
Gastrointestinal anthrax can occur when anthrax spores are ingested.

»What is transmission
by direct contact?

Direct contact transmission requires physical contact
between an infected person and a susceptible person, and the
physical transfer of microorganisms. Direct contact includes
touching an infected individual, kissing, sexual contact,
contact with oral secretions, or contact with body lesions.
This type of transmission requires close contact with an infected
individual, and will usually occur between members of the
same household or close friends and family.

Diseases spread exclusively by direct contact are
unable to survive for significant periods of time away from
a host. Sexually transmitted diseases are almost always spread
through direct contact, as they are extremely sensitive to
drying.

»What is transmission
by indirect contact?

Indirect contact transmission refers to situations
where a susceptible person is infected from contact with a contaminated
surface. Some organisms (such as Norwalk
Virus) are capable of surviving on surfaces for an extended
period of time. To reduce transmission by indirect contact, frequent
touch surfaces should be properly disinfected.

Frequent touch surfaces (fomites) include:

Door knobs, door handles, handrails

Tables, beds, chairs

Washroom surfaces

Cups, dishes, cutlery, trays

Medical instruments

Computer keyboards, mice, electronic devices with buttons

Pens, pencils, phones, office supplies

Children's toys

»What is transmission
by droplet contact?

Some diseases can be transferred by infected droplets
contacting surfaces of the eye, nose, or mouth. This is referred
to as droplet contact transmission. Droplets containing microorganisms
can be generated when an infected person coughs, sneezes, or talks.
Droplets can also be generated during certain medical procedures,
such as bronchoscopy. Droplets are too large to be airborne for
long periods of time, and quickly settle out of air.

Droplet transmission can be reduced with the use of
personal protective barriers, such as face masks and goggles.
Measles and SARS are examples
of diseases capable of droplet contact transmission.

»What is airborne transmission?

Airborne transmission refers to situations where droplet nuclei (residue from evaporated droplets) or dust particles containing microorganisms can remain suspended in air for long periods of time. These organisms must be capable of surviving for long periods of time outside the body and must be resistant to drying. Airborne transmission allows organisms to enter the upper and lower respiratory tracts. Fortunately, only a limited number of diseases are capable of airborne transmission.

Diseases capable of airborne transmission include:

Tuberculosis

Chickenpox

Measles

»What is fecal-oral transmission?

Fecal-oral transmission is usually associated with
organisms that infect the digestive system. Microorganisms enter
the body through ingestion of contaminated food and water. Inside
the digestive system (usually within the intestines) these microorganisms
multiply and are shed from the body in feces. If proper hygienic
and sanitation practices are not in place, the microorganisms
in the feces may contaminate the water supply through inadequate
sewage treatment and water filtration. Fish and shellfish that
swim in contaminated water may be used as food sources. If the
infected individual is a waiter, cook, or food handler, then inadequate
handwashing may result in food being contaminated with microorganisms.

»What is vector-borne
transmission?

Vectors are animals that are capable of transmitting
diseases. Examples of vectors are flies, mites, fleas, ticks,
rats, and dogs. The most common vector for disease is the
mosquito. Mosquitoes transfer disease through the saliva which
comes in contact with their hosts when they are withdrawing
blood. Mosquitoes are vectors for malaria, West
Nile virus, dengue fever, and yellow fever.

Vectors add an extra dimension to disease transmission.
Since vectors are mobile, they increase the transmission range
of a disease. Changes in vector behaviour will affect the
transmission pattern of a disease. It is important to study
the behaviour of the vector as well as the disease-causing
microorganism in order to establish a proper method of disease
prevention. In the case of malaria, insecticides were sprayed
and breeding grounds for mosquitoes were eliminated in an
attempt to control the spread of malaria.

Biting is not the only way vectors can transmit
diseases. Diseases may be spread through the feces of a vector.
Microorganisms could also be located on the outside surface
of a vector (such as a fly) and spread through physical contact
with food, a common touch surface, or a susceptible individual.

This website has been made possible through an unrestricted educational grant from
Pfizer Canada Inc.